At terahertz (THz) frequencies, electromagnetic (EM) fields can be absorbed by optically active internal vibrations of molecules. The capability of THz spectroscopy to detect directly the low-frequency vibrations of weak bonds, including but not limited to hydrogen bonds, is unique in providing information quite different from the visible or IR spectroscopic characterization. This uniqueness opens a large number of applications for THz vibrational spectroscopy in areas such as biomedicine, pharmaceutical analysis, real time monitoring of biological processes, detecting and identification of harmful biological species. A significant advantage of THz spectroscopy is that it is nondestructive to living species. Since each molecule has its own specific internal vibrations, this process can be used to fingerprint, characterize and identify a broad range of molecules. Very recently a THz spectroscopy technique for structural characterization of DNA, proteins and other bio-polymers in diluted solutions was developed by taking advantage of the lower water absorption in the sub-THz vs. IR and far IR regions [1-3].
However, several primary problems impede the development of THz spectroscopy of biological molecules and the application of this technique for characterization, detection, and discrimination between species as well as for the development of new devices for monitoring biological processes. The first problem is that the THz coupling to molecules is not very strong, resulting in poor sensitivity to molecular vibrations. The second problem is low spatial resolution due to the long wavelength of THz radiation (3 mm at 0.1 THz) and diffraction limitation. Thus, the spatial resolution is limited to several mm in the spectral range of 10-30 cm−1. This spectral range below 1 THz is especially attractive for practical applications because of low disturbance from the absorption by water or other solvents. In order to increase the sensitivity and reliability of THz fingerprinting techniques, coupling of incident THz radiation to biological or chemical molecules has to be enhanced.
The enhancement of the electric field was demonstrated long ago in optical diffraction by perfect metallic screens. Diffraction by a single slit in a perfect metallic screen was considered by Sommerfield [7]. He studied a case of the incident electromagnetic waves being normal to the screen and proved that the electric field is divergent at the edges of the slit if the incident electric field is perpendicular to the edges. Periodic slot arrays are other possible candidates for increasing the sensitivity. Such arrays were previously used for THz bandpass filters fabricated from lossy metal films deposited on dielectric membranes [8]. Experimental work on enhanced transmission are mostly available at optical and near-infrared frequencies for metallic periodic structures (gratings [9-12] and hole arrays [13-15]). Recently, it has been shown that waveguide resonance and diffraction are the main factors contributing to enhanced transmission of narrow slot subwavelength metallic gratings [12]. The phenomenon of extraordinary optical transmission (transmission efficiency exceeding unity when normalized to the surface of the holes) through hole arrays, first experimentally observed in Ag in 300 nm-1500 nm range [13-14], has been attributed to the resonant tunneling of surface plasmons [14-19] through thin films. Recently, similar studies were conducted in the THz range with hole arrays in films made of metals (Ag-coated stainless steel [20], Al-coated Si wafers [21]) and doped semiconductors (Si [22] and InSb [23]), and also with metallic slot arrays [24] using the perfect conductor approximation).